Neural event detection

ABSTRACT

A neural monitoring system for detecting an artificially-induced mechanical muscle response to an electrical stimulus at a stimulation frequency includes a mechanical sensor and a processor in communication with the mechanical sensor. The mechanical sensor is configured to be placed in mechanical communication with the muscle and to generate a mechanomyography output signal corresponding to a sensed mechanical movement of the muscle. The processor is configured to receive the mechanomyography output signal from the mechanical sensor and determine a frequency component of the mechanomyography output signal that has a peak magnitude and to detect the occurrence of an artificially-induced mechanical muscle response therefrom.

TECHNICAL FIELD

The present invention relates generally to a surgical diagnostic systemfor detecting the presence of one or more nerves.

BACKGROUND

Traditional surgical practices emphasize the importance of recognizingor verifying the location of nerves to avoid injuring them. Advances insurgical techniques include development of techniques including eversmaller exposures, such as minimally invasive surgical procedures, andthe insertion of ever more complex medical devices. With these advancesin surgical techniques, there is a corresponding need for improvementsin methods of detecting and/or avoiding nerves.

SUMMARY

A neural monitoring system is provided that is capable of detecting anartificially-induced mechanical response of a muscle to an electricalstimulus that is provided within an intracorporeal treatment area of ahuman subject. The electrical stimulus may be a periodic stimulusprovided at a stimulation frequency, and having a predetermined pulsewidth. Additionally, the intracorporeal treatment area generallyincludes a nerve that innervates the monitored muscle.

The neural monitoring system includes a non-invasive mechanical sensorand a processor. The non-invasive mechanical sensor is configured to beplaced in mechanical communication with the muscle and to generate amechanomyography output signal corresponding to a sensed mechanicalmovement of the muscle. By non-invasive, it is intended that themechanical sensor does not require an incision or related surgicalprocedure to be properly positioned. Instead, it may be held in contactwith an external surface of the skin, or may be positioned within anaturally occurring lumen/orifice. The mechanical sensor may generallyinclude an accelerometer, a microphone, a strain gauge, or apiezoelectric device.

The processor is in communication with the mechanical sensor and isconfigured to receive the mechanomyography output signal from themechanical sensor. In one configuration, the processor may determine afrequency component of the mechanomyography output signal that has apeak magnitude relative to one or more adjacent frequencies. Theprocessor may provide an indication to a user if the determinedfrequency component is either equal to, or an integer multiple of thestimulation frequency. In another configuration, the processor maydetermine a fundamental frequency of the mechanomyography output signal,and provide an indication to a user if the determined fundamentalfrequency is either equal to, or an integer multiple of the stimulationfrequency.

The above features and advantages and other features and advantages ofthe present invention are readily apparent from the following detaileddescription of the best modes for carrying out the invention when takenin connection with the accompanying drawings.

“A,” “an,” “the,” “at least one,” and “one or more” are usedinterchangeably to indicate that at least one of the item is present; aplurality of such items may be present unless the context clearlyindicates otherwise. All numerical values of parameters (e.g., ofquantities or conditions) in this specification, including the appendedclaims, are to be understood as being modified in all instances by theterm “about” whether or not “about” actually appears before thenumerical value. “About” indicates that the stated numerical valueallows some slight imprecision (with some approach to exactness in thevalue; about or reasonably close to the value; nearly). If theimprecision provided by “about” is not otherwise understood in the artwith this ordinary meaning, then “about” as used herein indicates atleast variations that may arise from ordinary methods of measuring andusing such parameters. In addition, disclosure of ranges includesdisclosure of all values and further divided ranges within the entirerange. Each value within a range and the endpoints of a range are herebyall disclosed as separate embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a neural monitoring system fordetecting an artificially-induced mechanical muscle response.

FIG. 2 is a schematic front view of the placement of a plurality ofmechanical sensors on the legs of a subject.

FIG. 3 is a schematic side view of an intracorporeal treatment areaincluding a portion of the lumbar spine.

FIG. 4 is a schematic time-domain graph of a mechnomyography outputsignal in response to a periodic electrical stimulus.

FIG. 5 is a schematic frequency-domain graph of a mechnomyography outputsignal in response to a periodic electrical stimulus.

FIG. 6 is a schematic flow diagram of a method for detecting anartificially-induced mechanical muscle response to an electricalstimulus.

FIG. 7 is a schematic flow diagram of a method of inferring anartificially-induced mechanical muscle response from a free-runningmechanomyography output signal.

DETAILED DESCRIPTION

Referring to the drawings, wherein like reference numerals are used toidentify like or identical components in the various views, FIG. 1schematically illustrates a neural monitoring system 10 that may be usedto identify the presence of one or more nerves within an intracorporealtreatment area 12 of a subject 14. As will be described in greaterdetail below, the system 10 may monitor one or more muscles of thesubject 14 for a mechanical motion, and may be capable of discriminatingan artificially-induced mechanical response of a muscle (also referredto as an “artificially-induced mechanical muscle response”) from asubject-intended muscle contraction/relaxation and/or an environmentallycaused movement. If an artificially-induced mechanical muscle responseis detected during the procedure, the system 10 may provide anindication to a user.

As used herein, an artificially-induced mechanical muscle responserefers to a contraction or relaxation of a muscle in response to astimulus that is not received through natural sensory means (e.g.,sight, sound, taste, smell, and touch). Instead, it is acontraction/relaxation of a muscle that is induced by the application ofa stimulus directly to a nerve that innervates the muscle. Examples ofstimuli that may cause an “artificially-induced” muscle response mayinclude an electrical current applied directly to the nerve or tointracorporeal tissue or fluid immediately surrounding the nerve. Inthis example, if the applied electrical current is sufficiently strongand/or sufficiently close to the nerve, it may artificially cause thenerve to depolarize (resulting in a corresponding contraction of themuscle innervated by that nerve). Other examples of such “artificialstimuli” may involve mechanically-induced depolarization (e.g.,physically stretching or compressing a nerve, such as with a tissueretractor), thermally-induced depolarization (e.g., through ultrasoniccautery), or chemically-induced depolarization (e.g., through theapplication of a chemical agent to the tissue surrounding the nerve).

During an artificially-induced mechanical muscle response, a muscleinnervated by the artificially depolarized nerve may physically contractor relax (i.e., a mechanical response). Such a mechanical reaction mayprimarily occur along a longitudinal direction of the muscle (i.e., adirection aligned with the constituent fibers of the muscle), though mayfurther result in a respective swelling/relaxing of the muscle in alateral direction (which may be substantially normal to the skin formost skeletal muscles). This local movement of the muscle during anartificially-induced mechanical muscle response may be measured relativeto the position of the muscle when in a non-stimulated state, and isdistinguished from other global translations of the muscle

The neural monitoring system 10 may include a processor 20 that is incommunication with at least one mechanical sensor 22. The mechanicalsensor 22 may include, for example, a strain gauge, a force transducer,a position encoder, an accelerometer, a piezoelectric material, or anyother transducer or combination of transducers that may convert aphysical motion into a variable electrical signal.

Each mechanical sensor 22 may specially be configured to monitor a localmechanical movement of a muscle of the subject 14. For example, eachsensor 22 may include a fastening means, such as an adhesivematerial/patch, that allows the sensor 22 to be adhered, bandaged, orotherwise affixed to the skin of the subject 14 (i.e. affixed on anexternal skin surface). Other examples of suitable fastening means mayinclude bandages, sleeves, or other elastic fastening devices that mayhold the sensor 22 in physical contact with the subject 14.Alternatively, the mechanical sensor 22 (and/or coupled device) may beconfigured to monitor a local mechanical movement of a muscle by virtueof its physical design. For example, the sensors/coupled devices mayinclude catheters, balloons, bite guards, orifice plugs or endotrachealtubes that may be positioned within a lumen or natural opening of thesubject to monitor a response of the lumen or orifice, or of a musclethat is directly adjacent to and/or connected with the lumen or orifice.In one configuration, the mechanical sensor may be a non-invasivedevice, whereby the term “non-invasive” is intended to mean that thesensor is not surgically placed within the body of the subject (i.e.,via cutting of tissue to effectuate the placement). For the purposes ofthis disclosure, non-invasive sensors may include sensors that areplaced within naturally occurring body lumens that are accessiblewithout the need for an incision.

In one configuration, the sensor 22 may include a contact detectiondevice, that may provide an indication if the sensor 22 is in physicalcontact with the skin of the subject 14. The contact detection devicemay, for example, include a pair of electrodes that are configured tocontact the skin of the subject 14 when the sensor 22 is properlypositioned. The sensor 22/contact detection device may then monitor animpedance between the electrodes to determine whether the electrodes arein contact with the skin. Other examples of suitable contact detectiondevices may include capacitive touch sensors or buttons that protrudeslightly beyond the surface of the sensor.

The system 10 may further include one or more elongate medicalinstruments 30 that are capable of selectively providing a stimuluswithin the intracorporeal treatment area 12 of the subject 14 (i.e.,also referred to as a stimulator 30). For example, in one configuration,the elongate medical instrument 30 may include a probe 32 (e.g., aball-tip probe, k-wire, or needle) that has an electrode 34 disposed ona distal end portion 36. The electrode 34 may be selectivelyelectrified, at either the request of a user/physician, or at thecommand of the processor 20, to provide an electrical stimulus 38 tointracorporeal tissue of the subject. In other configurations, theelongate medical instrument 30 may include a dialator, retractor, clip,cautery probe, pedicle screw, or any other medical instrument that maybe used in an invasive medical procedure. Regardless of the instrument,if the intended artificial stimulus is an electrical current, theinstrument 30 may include a selectively electrifiable electrode 34disposed at a portion of the instrument that is intended to contacttissue within the intracorporeal treatment area 12 during a procedure.

During a surgical procedure, the user/surgeon may selectively administerthe stimulus to intracorporeal tissue within the treatment area 12 toidentify the presence of one or more nerve bundles or fibers. For anelectrical stimulus 38, the user/surgeon may administer the stimulus,for example, upon depressing a button or foot pedal that is incommunication with the system 10, and more specifically in communicationwith the stimulator 30. The electrical stimulus 38 may, for example, bea discrete pulse (e.g., a step pulse) having a pulse width within therange of about 30 μs to about 500 μs. In other examples, the discretepulse may have a pulse width within the range of about 50 μs to about200 μs, or within the range of about 75 μs to about 125 μs. The discretepulse may be periodically applied at a frequency of, for example,between about 1 Hz and about 10 Hz.

If a nerve extends within a predetermined distance of the electrode 34,the electrical stimulus 38 may cause the nerve to depolarize, resultingin a mechanical twitch of a muscle that is innervated by the nerve(i.e., an artificially-induced mechanical muscle response). In general,the magnitude of the response/twitch may be directly correlated to thedistance between the electrode and the nerve, and the magnitude of thestimulus current. In one configuration, a lookup table may be employedby the processor 20 to provide an approximate distance between theelectrode and the nerve, given a known stimulus magnitude and a measuredmechanical muscle response.

Prior to beginning a surgical procedure, the one or more mechanicalsensors 22 may be placed in mechanical communication with one or moremuscles of the subject 14. In the present context, a sensor 22 may be inmechanical communication with the muscle if it can physically detect amovement, velocity, acceleration, strain or other physical response ofthe muscle, either via direct contact with the muscle, or via amechanical relationship through one or more intermediate materialsand/or tissues (e.g., skin and/or subcutaneous tissue).

FIG. 2 illustrates an example of the placement of a plurality ofmechanical sensors 22 for a surgical procedure that may occur proximatethe L2, L3, and/or L4 vertebrae of the lumbar spine (shown schematicallyin FIG. 3). The nerves 50, 52 and 54 exiting the L2, L3 and L4 foramen56, 58, 60 may therefore either lie within the treatment area 12 (i.e.,the area surrounding the L2, L3, and/or L4 vertebrae), or may beimmediately proximate to this area. Using common anatomical knowledge,the surgeon may understand that damage to these nerves 50, 52, 54 mayaffect the functioning of the vastus medialis muscles and the tibialisanterior muscles. As such, the surgeon may place mechanical sensors 22a-22 d on or near the vastus medialis muscles and the tibialis anteriormuscles to guard against inadvertent manipulation of the nerves duringthe procedure. For example, mechanical sensors 22 a and 22 b are placedon the vastus medialis muscles, which are innervated by the nerves 50,52 exiting the L2 and L3 foramen 56, 58, and sensors 22 c and 22 d areplaced on the tibialis anterior muscles, which are innervated by thenerves 54 exiting the L4 foramen 60.

In general, each mechanical sensor 22 may generate a mechanomyography(MMG) output signal (schematically shown at 62) that corresponds to asensed mechanical movement/response of the adjacent muscle. The MMGoutput signal 62 may be either a digital or analog signal, and maytypically be provided to the processor 20 through either wired orwireless communication means (e.g., through a physical wire, or usingradio frequency communication protocols, such as according to IEEE802.11 or another protocol such as Bluetooth). As a specific signal, theMMG output signal 62 is intended to be separate and distinct from anyelectrical potentials of the muscle or skin (often referred to aselectromyography (EMG) signals). While electrical (EMG) and mechanical(MMG) muscle responses may be related, their relationship is complex,and not easily described (e.g., electrical potentials are very locationspecific, with a potentially variable electrical potential across thevolume of the muscle of interest).

Referring again to FIG. 1, the processor 20 may be in communication withthe stimulator 30 and the mechanical sensor 22, and may be configured toreceive the MMG output signal 62 from the mechanical sensor 22. Theprocessor 20 may be embodied as one or multiple digital computers, dataprocessing devices, and/or digital signal processors (DSPs), which mayhave one or more microcontrollers or central processing units (CPUs),read only memory (ROM), random access memory (RAM),electrically-erasable programmable read only memory (EEPROM), ahigh-speed clock, analog-to-digital (A/D) circuitry, digital-to-analog(D/A) circuitry, input/output (I/O) circuitry, and/or signalconditioning and buffering electronics.

The processor 20 may be configured to automatically perform one or moresignal processing algorithms 70 or methods to determine whether a sensedmechanical movement (i.e., via the MMG output signal 62) isrepresentative of an artificially-induced mechanical muscle response orif it is merely a subject-intended muscle movement and/or anenvironmentally caused movement. These processing algorithms 70 may beembodied as software or firmware, and may either be stored locally onthe processor 20, or may be readily assessable by the processor 20.

FIG. 4 generally illustrates a graph 80 of an MMG output signal 82 inresponse to a periodic electrical stimulus 38 provided proximate to anerve. This MMG output signal 82 may be in response to an electricalstimulus 38 that is provided at about a 3 Hz frequency. As shown, theMMG output signal 82 has an amplitude 84 that varies as a function oftime 86 and includes a plurality of generally discrete contractionevents 88. Each contraction event 88 may include, for example, aninitial response 90, and a plurality of subsequent peaks/valleys 92 thatare smaller than the initial response 90.

While the muscle contractions may be easily represented in the timedomain (as generally illustrated by the graph 80 in FIG. 4), they mayalso be represented in the frequency domain (as generally illustrated bythe graph 94 in FIG. 5). In general, the frequency domain represents asignal as a plurality of discrete frequency components 96, each having arespective magnitude 98. It has been found that an induced muscleresponse has identifiable frequency content in the range of about 0 Hzto about 50 Hz, with a majority of the signal energy being in the rangeof about 0 Hz to about 20 Hz.

FIGS. 6 and 7 generally illustrate two methods 100, 200 for detecting anartificially-induced mechanical muscle response, both occurring in thefrequency domain. In general, the method 100 provided in FIG. 6 mayoperate by correlating the MMG output signal 62 in the frequency domainto an applied electrical stimulus 38. Conversely, the method 200provided in FIG. 7 may operate by solely monitoring the MMG outputsignal 62 in the frequency domain (i.e., a “free-run” MMG monitoringtechnique). The two provided methods 100, 200 may be used individually,or may be combined together to provide a more robust detection scheme.

The flow diagram of FIG. 6 generally illustrates a method 100 fordetecting an artificially-induced mechanical muscle response to anelectrical stimulus 38 that is provided proximate to a nerve. As shown,the method 100 begins when the processor 20 receives the MMG outputsignal 62 from the mechanical sensor 22 (at step 102). The MMG outputsignal 62 may be received as a time domain signal, and may potentiallybe filtered (either by circuitry associated with the mechanical sensor22 or processor 20, or through digital filtering techniques performed bythe processor 20) to remove high frequency noise (such filtering isshown by example in FIG. 4 via the two data traces). For example, in oneconfiguration, the MMG output signal 62 may be filtered using a low passfilter having a cutoff frequency greater than about 200 Hz. In otherembodiments, a low pass filter may be used that has a cut off frequencygreater than about 50 Hz, or greater than about 20 Hz, or in the rangeof about 20 Hz to about 50 Hz. In addition to filtering through a lowpass filter, the MMG output signal 62 may be further filtered by a highpass filter. In one configuration, the high pass filter may have a cutoff frequency that is greater than about 0 Hz though less than about 10Hz.

After the processor 20 receives the MMG output signal 62 it may convertthe received signal 62 from the time domain to the frequency domain (atstep 104). Such a conversion may occur using, for example, FourierMethods (e.g., a Fourier Transform, a Fast Fourier Transform, or aDiscrete Fourier Transform), or through other similar methodologies.Once in the frequency domain, the processor 20 may then determine one ormore frequency components that have a peak magnitude (at step 106).While numerous methods may be used to detect magnitude peaks, the mostbasic method includes identifying one or more frequencies that havemagnitudes greater than the magnitudes of adjacent frequencies. FIG. 5generally illustrates at least 6 detectable peaks/peak magnitudes 150.As shown, these peaks 150 correspond to frequencies of about 3 Hz, about6 Hz, about 9 Hz, about 12 Hz, about 15 Hz, and about 18 Hz.

Referring again to FIG. 6, once the MMG output signal 62 has beenconverted into the frequency domain (step 104) and a subset offrequencies that correspond to magnitude peaks have been identified(step 106), the processor 20 may determine whether anartificially-induced mechanical muscle response has been sensed by themechanical sensor 22 (at step 108). If the processor 20 detects such aninduced muscle response, then it may provide an indication to a usercorresponding to the detected event (at step 110). In one configuration,this indication may include one or more of an illuminated light/LED, acolored light/LED, a textual or symbolic alert on a display deviceassociated with the processor 20, a vibration in the handle of thestimulator, and an audible alert such as a single frequency alert, amultiple frequency alert, and/or an audible natural language alert.Moreover, the indication/alert may include an estimation of theproximity between the electrode and the nerve, such as may be derivedusing a lookup table as described above, or as explained in U.S. Pat.No. 8,343,065 to Bartol, et al., entitled “NEURAL EVENT DETECTION,”which is hereby incorporated by reference in its entirety and for all ofthe disclosure setforth therein.

Referring back to step 108, in general, there may be two strategies(112, 114) that the processor 20 may employ to determine whether asensed movement of the mechanical sensor 22 is indicative of anartificially-induced mechanical muscle response. These strategies 112,114 may both operate by attempting to correlate the detected frequencypeaks of the MMG output signal with an attribute of the providedelectrical stimulus 38. In practice, the processor 20 may use either ofthese strategies alone, or it may combine them both together into asingle strategy (i.e., either performed concurrently, sequentially, orin combination).

In the first detection strategy 112, the processor 20 may receive anindication of the frequency at which the electrical stimulus 38 isadministered (at 118). This frequency indication may either be receiveddirectly from the stimulator, or from a register or memory locationwithin the processor itself. In step 120, the processor 20 may examinethe frequencies corresponding to magnitude peaks from step 106, anddetermine whether any of the identified frequencies is an integermultiple of the stimulation frequency. Finally, in step 122 theprocessor 20 may identify that the sensed response is indicative of anartificially-induced mechanical muscle response if it is determined thatone or more of the identified frequencies is an integer multiple of thestimulation frequency. In other configurations, this determination mayrequire that two or more, or even three or more frequencies are integermultiples of the stimulation frequency before an artificially-inducedmechanical muscle response is identified.

In the second detection strategy 114, the processor 20 may similarlyreceive an indication of the frequency at which the electrical stimulus38 is administered (at 118). In step 124, the processor 20 may examinethe frequencies corresponding to magnitude peaks from step 106, anddetermine a fundamental frequency of the MMG output signal 62. This maybe accomplished, for example, by determining a greatest common factor ofa plurality of the frequencies where magnitude peaks are detected. Thistechnique applied to the peaks 150 shown in FIG. 6 would result in afundamental frequency of 3 Hz (i.e., a common factor of 3, 6, 9, 12, 15,and 18). In most cases, the determined fundamental frequency may liewithin the range of about 1 Hz to about 10 Hz (i.e., the typical rangeof simulation frequencies). In step 126, the processor 20 may determinewhether the fundamental frequency is either equal to, or an integermultiple of the stimulation frequency, and if so, in step 122, theprocessor 20 may identify that the sensed response is indicative of anartificially-induced mechanical muscle response.

The method 100 illustrated in FIG. 6 may generally provide ahigh-confidence indication that a nerve is proximate to the stimulatorby correlating an applied stimulus with a monitored response. In someprocedures, however, the use of a stimulator may either be impractical,or may not adequately assess the risks posed to nerves that are furtheraway. For example, while stimulated detection may be extremely usefulwhen making a lateral transpsoas approach to the spine, the subsequentuse of a mechanical retractor to provide an operating corridor mayaffect nerves that are outside of the electrically-stimulated detectionradius. In this manner it may also be useful to have a free-runningdetection algorithm that may monitor for artificially-induced mechanicalmuscle responses that are attributable to, for example, mechanicalstimuli. One embodiment of a free-running detection algorithm is shownby the method 200 provided in FIG. 7. While this method 200 may onlyprovide an “inference” of an artificially-induced mechanical muscleresponse (i.e., due to the absence of a direct correlation), it may beequally valuable in some circumstances.

As shown in FIG. 7, the free-running detection method 200 may be similarto the method 100 of FIG. 6, though may make detection inferenceswithout the knowledge of a stimulation frequency. As shown, the method200 begins when the processor 20 receives the MMG output signal 62 fromthe mechanical sensor 22 (at step 102), and subsequently converts theMMG output signal 62 into the frequency domain (step 104). In step 202,the processor 20 may determine (i.e., infer) whether sensed motiondetected by the mechanical sensor 22 is indicative of anartificially-induced mechanical muscle response. If the processor 20does determine that such an induced muscle response has occurred, it maythen provide an indication to a user of such an event (at step 110).

Referring back to step 202, in general, there may be three free-rundetection strategies (204, 206, 208) that the processor 20 may employ todetermine/infer whether a sensed movement of the mechanical sensor 22 isindicative of an artificially-induced mechanical muscle response. Thesefree-run strategies 204, 206, 208 may operate by monitoring the MMGoutput signal, and attempting to detect signal attributes that may beindicative of an artificially-induced mechanical muscle response. Inpractice, the processor 20 may use any of these strategies alone, or itmay combine two or more of them together into a single strategy (i.e.,either performed concurrently, sequentially, or in combination, and/ormay be combined with the stimulated techniques of FIG. 6).

In the first free-run detection strategy 204, the processor 20 may firstdetermine one or more frequency components that have a peak magnitude(at step 106). In step 124, the processor 20 may examine the frequenciescorresponding to magnitude peaks from step 106, and determine afundamental frequency of the MMG output signal 62. Finally, theprocessor 20 may compare the determined fundamental frequency to a rangeof expected fundamental frequencies for an artificially-inducedmechanical muscle response in step 210. Such a range may be, forexample, between about 1 Hz and about 10 Hz. If the determinedfundamental frequency falls within this range, the processor 20 mayinfer that the sensed mechanical sensor movement is indicative of anartificially-induced mechanical muscle response (in step 212).

In the second free-run detection strategy 206, the processor 20 maybegin by determining one or more frequency components that have a peakmagnitude (at step 106). Once the peaks are identified, the processor 20may compare the peaks to a range of frequencies where frequency contentis expected to exist for an artificially-induced mechanical muscleresponse (in step 214). Such a range may be, for example between about 1Hz and about 20 Hz. In one configuration, if one or more of theidentified peaks are within this range, the processor 20 may infer thatthe sensed response is indicative of an artificially-induced mechanicalmuscle response in step 212. In other configurations, the processor 20may require that two or more, or even three or more of the identifiedpeaks lie within the range before it infers that the sensed response isindicative of an artificially-induced mechanical muscle response.

Finally, in the third free-run detection strategy, 208 the processor 20does not necessarily need to compute the frequencies corresponding tomagnitude peaks, instead, it may first establish a noise floor at step216, and then it may determine if any of the respective frequencymagnitudes exceed the noise floor by a threshold amount (in step 218).If so, the processor 20 may infer that the sensed response is indicativeof an artificially-induced mechanical muscle response in step 212.

The noise floor may generally represent the normal background mechanicalnoise/movement that may be reported by the sensor. It may be a functionof the precision of the transducer within the sensor, it may includereceived electromagnetic radiation, and/or it may include mechanicalmovement that may be attributable to continuous rhythmic events such asbreathing or heart beat. The noise floor may either have a varyingmagnitude for each frequency, or may generally be a constant valueacross all frequencies. In this strategy 208, the threshold may beeither a fixed amount above the noise floor, or may be a multiple of thenoise floor (e.g., a level twice the noise floor, or a level that is setabout one or more standard deviations above an average noise levelacross a period of time).

While the stimulation-based methods/strategies of FIG. 6, as well as thegraphs in FIGS. 4 and 5 generally illustrate the present detectionschemes using only a single simulation frequency, thesemethods/strategies may be expanded to the use of multiple stimulationfrequencies as well. For example, if a stimulator were to provide anelectrical stimulus having both a 3 Hz component and a 5 Hz component,an MMG output signal that is characteristic of an induced muscleresponse may include peaks at 3 hz, 5 hz, 6 hz, 9 hz, 10 hz, 12 hz, 15hz within the frequency domain. Also, peaks may be apparent at multiplesof the difference in the frequencies (i.e., 2 Hz, 4 Hz, 8 Hz, etc)

In addition to use as a stand alone, or hand-held nerve monitoringapparatus, the present nerve monitoring system 10 and describedartificially-induced mechanical muscle response detection algorithms (asdescribed within method 100) may be used by a robotic surgical system,such as described in U.S. patent application Ser. No. 13/428,693, filed23 Mar. 2012, entitled “ROBOTIC SURGICAL SYSTEM WITH MECHANOMYOGRAPHYFEEDBACK,” which is incorporated by reference in its entirety and forall of the disclosure setforth therein. In such a system, theabove-described neural monitoring system 10 may be used to provide oneor more control signals to a robotic surgical system if anartificially-induced mechanical muscle response is detected. In such anembodiment, the one or more elongate medical instruments 30 describedabove may be robotically controlled in up to 6 or more degrees offreedom/motion by a robotic controller. This instrument may beconfigured to perform a surgical procedure within an intracorporealtreatment area at the direction of the robotic controller, and mayprovide an electrical stimulus 38 in the manner described above. If anartificially-induced mechanical muscle response is detected, the neuralmonitoring system 10 may instruct the robotic controller (via theprovided control signal) to limit the range of available motion of theelongate medical instrument 30 and/or to prevent an actuation of an endeffector that may be disposed on the instrument 30 and controllable bythe robotic controller.

While the best modes for carrying out the invention have been describedin detail, those familiar with the art to which this invention relateswill recognize various alternative designs and embodiments forpracticing the invention within the scope of the appended claims. It isintended that all matter contained in the above description or shown inthe accompanying drawings shall be interpreted as illustrative only andnot as limiting.

The invention claimed is:
 1. A neural monitoring system for detecting anartificially-induced mechanical response of a muscle to an electricalstimulus provided within an intracorporeal treatment area of a humansubject at a stimulation frequency, the intracorporeal treatment areaincluding a nerve that innervates the muscle, the neural monitoringsystem comprising: a non-invasive mechanical sensor configured to beplaced in mechanical communication with the muscle and to generate amechanomyography output signal corresponding to a sensed mechanicalmovement of the muscle; and a processor in communication with themechanical sensor and configured to: receive the mechanomyography outputsignal from the mechanical sensor; determine a frequency component ofthe mechanomyography output signal, wherein the frequency component hasa peak magnitude relative to adjacent frequencies; and provide anindication of an artificially-induced mechanical muscle response to auser if the determined frequency component is either equal to, or aninteger multiple of the stimulation frequency.
 2. The neural monitoringsystem of claim 1, wherein the processor is further configured toattenuate frequency content of the mechanomyography output signal thatis above a cutoff frequency.
 3. The neural monitoring system of claim 2,wherein the cutoff frequency is in the range of about 20 Hz to about 50Hz.
 4. The neural monitoring system of claim 1, wherein the processor isconfigured to determine a frequency component of the mechanomyographyoutput signal that has a peak magnitude by: converting the receivedmechanomyography output signal from a time domain to a frequency domain,wherein the frequency domain is characterized by a plurality of discretefrequencies, each having a corresponding magnitude; and identifying thefrequency component from the plurality of discrete frequencies thatcorresponds to a magnitude peak.
 5. The neural monitoring system ofclaim 1, wherein the frequency component is within the range of greaterthan 0 Hz to about 20 Hz.
 6. The neural monitoring system of claim 1,wherein the mechanical sensor includes an accelerometer, a microphone, astrain gauge, or a piezoelectric device.
 7. The neural monitoring systemof claim 1, wherein the frequency component is a fundamental frequencyof the mechanomyography output signal.
 8. The neural monitoring systemof claim 1, further comprising an elongate medical instrument that isconfigured to selectively provide the electrical stimulus within theintracorporeal treatment area of the subject at the stimulationfrequency.
 9. The neural monitoring system of claim 8, wherein theelectrical stimulus includes a periodic pulse having a pulse widthbetween about 30 μs and about 500 μs.
 10. A neural monitoring system fordetecting an artificially-induced mechanical response of a muscle to anelectrical stimulus provided within an intracorporeal treatment area ofa human subject at a stimulation frequency, the intracorporeal treatmentarea including a nerve that innervates the muscle, the neural monitoringsystem comprising: a non-invasive mechanical sensor configured to beplaced in mechanical communication with the muscle and to generate amechanomyography output signal corresponding to a sensed mechanicalmovement of the muscle; and a processor in communication with themechanical sensor and configured to: receive the mechanomyography outputsignal from the mechanical sensor; determine a fundamental frequency ofthe mechanomyography output signal; provide an indication of anartificially-induced mechanical muscle response to a user if thedetermined fundamental frequency is either equal to, or an integermultiple of the stimulation frequency.
 11. The neural monitoring systemof claim 10, wherein the processor is further configured to attenuatefrequency content of the mechanomyography output signal that is above acutoff frequency.
 12. The neural monitoring system of claim 11, whereinthe cutoff frequency is in the range of about 20 Hz to about 50 Hz. 13.The neural monitoring system of claim 10, wherein the processor isconfigured to determine a fundamental frequency of the mechanomyographyoutput signal by: converting the received mechanomyography output signalfrom a time domain to a frequency domain, wherein the frequency domainis characterized by a plurality of discrete frequencies, each having acorresponding magnitude; identifying a subset of the plurality ofdiscrete frequencies wherein each discrete frequency belonging to thesubset corresponds to a respective magnitude peak; and determining agreatest common factor of the frequencies belonging to the subset. 14.The neural monitoring system of claim 10, wherein the fundamentalfrequency is within the range of greater than 0 Hz to about 10 Hz. 15.The neural monitoring system of claim 10, wherein the mechanical sensorincludes an accelerometer, a microphone, a strain gauge, or apiezoelectric device.
 16. The neural monitoring system of claim 10,further comprising an elongate medical instrument that is configured toselectively provide the electrical stimulus within the intracorporealtreatment area of the subject at the stimulation frequency.
 17. Theneural monitoring system of claim 16, wherein the electrical stimulusincludes a periodic pulse having a pulse width between about 30 μs andabout 500 μs.
 18. A neural monitoring system for detecting anartificially-induced mechanical response of a muscle to an electricalstimulus provided within an intracorporeal treatment area of a humansubject at a stimulation frequency, the intracorporeal treatment areaincluding a nerve that innervates the muscle, the neural monitoringsystem comprising: a non-invasive mechanical sensor configured to beplaced in mechanical communication with the muscle and to generate amechanomyography output signal corresponding to a sensed mechanicalmovement of the muscle; and a processor in communication with themechanical sensor and configured to: receive the mechanomyography outputsignal from the mechanical sensor; determine a frequency component ofthe mechanomyography output signal, wherein the frequency component hasa peak magnitude relative to adjacent frequencies; and provide anindication of an artificially-induced mechanical muscle response to auser if the determined frequency component is either equal to, or aninteger multiple of the stimulation frequency, and either: thedetermined frequency component is within the range of greater than 0 Hzto about 20 Hz; or the peak magnitude is above an established noisefloor by a threshold amount.
 19. The neural monitoring system of claim18, wherein the mechanical sensor includes an accelerometer, amicrophone, a strain gauge, or a piezoelectric device.